1. Field of the Invention
The present invention relates to an estimation method for a residual discharging time of batteries (i.e. Li—Fe batteries). More particularly, the present invention relates to an estimation method for a residual discharging time of batteries utilizing multi-level Peukert equations. When a discharge current is detected, a residual discharging time is estimated by the Peukert equation of the discharge current interval corresponding to the detected discharge current, and discharge times are added up while discharging in a corresponding discharge current interval. If a discharge voltage is lower than a predetermined voltage, an estimation error of the residual discharging time is calculated. When the estimation error is greater than a predetermined value, parameters of the corresponding multi-level Peukert equation are adjusted.
2. Description of the Related Art
Recently, global warming has resulted in climate changes due to massive carbon dioxide release via the burning of fossil fuels. Thus, renewal energies (i.e. wind energy and solar energy) have been developed. In order to effectively manage and save electric power, high-capacity energy storage elements are provided. Currently, the widely-used high-capacity energy storage elements are lead-acid batteries. Even though the lead-acid batteries have advantages of low cost and high battery capacity, this type of battery cannot be charged by a high current for reducing the charging time. In addition, a high-current discharge will also reduce the battery capacity of the lead-acid batteries.
Li—Fe batteries provide a high-current charging/discharging operation which cannot be achieved by lead-acid batteries. Furthermore, a high-current discharge may not obviously affect the battery capacity of the Li—Fe batteries. The Li—Fe batteries further have advantages of high safety and stability in use. Accordingly, Li—Fe batteries are suitably used as high-capacity energy storage elements.
In order to improve the reliability and safety of battery and electric energy usage, there is a need for precisely estimating a residual discharging time of batteries. By way of example, U.S. Pat. No. 4,595,880, entitled “Battery State of Charge Gauge,” discloses an ampere-hour method utilized to sense and sum up a real battery-discharging capacity for computing an average discharge current within a predetermined time unit. The used battery capacity is calculated from the average discharge current using the Peukert equation. Consequently, the discharge state of batteries, as well as the residual discharge capacity of batteries, is calculated from the used battery capacity using a function of the state of battery charge.
However, the above estimation method for a residual discharge capacity of batteries, disclosed in U.S. Pat. No. 4,595,880, requires computing an average discharge current within a predetermined time unit, and calculating the used battery capacity which is calculated from the average discharge current using the Peukert equation. However, the parameters of the Peukert equation must be obtained from experimental data, and cannot be precisely adjusted in response to conditions of battery's temperature change and battery aging, for example. However, once the changes of the battery's discharge current and temperature are large, the above estimation method cannot be operated to precisely adjust the parameters of the Peukert equation. Disadvantageously, this may result in a greater estimation error. Furthermore, the above estimation method fails to provide a function of estimating a residual discharging time of batteries.
U.S. Pat. No. 5,761,072, entitled “Battery State of Charge Sensing System,” discloses two sets of average discharge currents are calculated from discharge currents of batteries using two predetermined functions. Two battery capacities and weighting constants thereof are calculated from the average discharge currents using the Peukert equation. Finally, the discharge state of batteries, as well as the residual discharge capacity of batteries, is calculated from the real battery capacities and weighting constants using a function of the discharge state of batteries.
However, the above estimation method for a residual discharge capacity of batteries, disclosed in U.S. Pat. No. 5,761,072, requires calculating two different average discharge currents from a battery's characteristic, and calculating battery capacities from the weighting constants. The discharge reaction of batteries is generally a chemical reaction. Therefore, the discharge current calculated from a single set of the battery's characteristic couldn't completely reflect the total effect of a real battery's characteristic. In fact, an operation manner, a discharge depth, an environmental temperature or other external factors may affect the discharge capacity of batteries. The above estimation method utilizes a single set of the Peukert equation to calculate the discharge capacity of batteries that may susceptibly result in an estimation error under different discharge currents. Furthermore, the above estimation method fails to automatically adjust the estimation error and to provide a function of estimating a residual discharging time of batteries.
Briefly, there is a need of improving the estimation method for a residual discharge capacity of batteries disclosed in U.S. Pat. No. 4,595,880 and U.S. Pat. No. 5,761,072 for reducing the estimation error. However, the estimation method using the Peukert equation also fails to provide a function of estimating a residual discharging time of batteries.
Taiwanese Patent Appl. Publication No. 200905230, entitled “Estimating Method for Battery Residue Capacity,” discloses a method, including the steps of: providing a plurality of battery-discharge-current intervals and a plurality of Peukert equations, with each Peukert equation including parameters; detecting at least one discharge current of batteries; selecting the Peukert equation which corresponds to the associated battery-discharge-current interval; calculating an estimation value of battery capacity and a battery residue capacity; calculating a capacity correction factor for the designated battery-discharge-current interval if necessarily changing the battery-discharge-current interval; and recalculating the parameters of the Peukert equation if an error value between the estimation value and the real value of discharge capacity of batteries is greater than a predetermined value.
However, the above estimation method, disclosed in TWN Pub. No. 200905230, only provides a correction for the parameters of the Peukert equation in estimating the residue capacity of batteries, and fails to provide a correction for the parameters for estimating a residual discharging time of batteries. Hence, there is a need of providing a correction in estimating a residual discharging time of batteries.
As is described in greater detail below, the present invention provides an estimation method for a residual discharging time of batteries utilizing multi-level Peukert equations. When an estimation error of the residual discharging time is greater than a predetermined value, associated parameters of the multi-level Peukert equations are adjusted in such a way to mitigate and overcome the above problem.